It is known that the backscatter coefficient from a radar wave transmitted at radiofrequency from a satellite and towards the sea can be measured in order to determine the speeds and the directions of winds at sea, with this so-called "Bragg" backscattering being highly influenced by wind in the short term.
In the short term, wind creates small waves or "crinkles" having a physical wavelength of the order of a few centimeters.
By using a radar wave transmitted from a satellite and of approximately the same wavelength, a resonance phenomenon is set up enabling the backscatter coefficient of the sea to be measured, which coefficient is itself highly dependent on short term wind.
To measure wind direction, it is necessary to provide a plurality of antennas on the satellite pointing along different azimuths, with there generally being two or three such antennas per "swath" (observation strip).
Most known apparatuses provide two or three antennas per swath, generally:
a "front" antenna whose track on the Earth is directed at an angle of 45.degree. forwards from the satellite;
a "middle" antenna whose track on the Earth is perpendicular to the Earth track of the satellite; and
a "rear" antenna whose track on the Earth is directed symmetrically to the track of the front antenna relative to the track of the middle antenna, i.e. at 45.degree. towards the rear.
Consequently, any given point on the swath is measured on three successive occasions at different angles of azimuth and incidence, these three occasions making it possible to resolve ambiguities between the direction and the amplitude of the short term wind at said point.
It is not absolutely essential to use three measurements per swath, and, for example, the American satellite "Seasat" uses only two, however that suffers from the drawback of making it more difficult to resolve the above-mentioned ambiguity concerning wind direction.
In addition to the "Seasat" implementation, the following known systems should be mentioned:
the "N Scatt" project designed for band Ku operation like the Seasat implementation, but using three measurements per swath;
the European Space Agency's current "ERS-1" and "ERS-2" implementations under the general name "A.M.I." that use three antennas per swath in association with short pulses that require high transmission power, with those two implementations sharing with a synthetic aperture radar (SAR) operating in band C; and
SCATT-2 and two-swath AMI-2 projects which constitute improvements over the above systems while still not being optimal.
Apart from the two last-mentioned systems, those prior art systems require the use of high-power transmitter amplifiers using vacuum tubes, which are particularly bulky and unreliable. In addition, they are poorly or badly optimized for the function of measuring scatter. For example, the ERS-1 system requires a compromise to be made between radiometric resolution and three-dimensional resolution, and this compromise is achieved to the detriment of three-dimensional resolution. Since that system is also shared with an imaging SAR, it is not optimized for the scatter-measuring function, and it has a limited utilization rate. The use of vacuum tube amplifiers having high peak powers (5 kilowatts or more) gives rise to poor reliability and to risks of gaseous discharges or the "multipactor" effect in the waveguides associated with said amplifiers.
The present invention seeks to remedy these drawbacks and it provides satellite radar apparatus for measuring the backscatter coefficient of the sea to determine the speeds and the directions of winds at sea (with such apparatus commonly being called a "wind scatter meter"), the apparatus using three aiming directions per swath and being much better optimized than presently known apparatuses with respect to cost, performance, and platform resources (mass, power consumption), with the sole barriers that remain in practice being essentially due to the laws of physics and to the state of advance of technology.